Thancred used to fight with a dagger, no shield, as a Gladiator. Then they added Rogue and suddenly he's dual-wielding daggers and performing feats of acrobatics we had not seen prior to that. Alisae was introduced to us as an Arcanist, holding a book much like her brother does. Then she summons a sword and starts combining swordsmanship with magic. Somehow you seem to think this constitutes a leap of logic. It's called, 'Speculation,' and, 'Deductive Reasoning.' Even the twins make note that while Alphinaud is a back-line combatant Alisae prefers to get up close and personal with her foes, not unlike how we've seen Red Mages will close gaps to make melee strikes.

But like I said, please do enlighten us as to which job you think Alisae is. I'd love to hear your reasoning.